COLORADO’S STRATIGRAPHY RE-CHARTED, REVEALS PATTERNS

We have compiled a revision of Richard Pearl’s 1974 Colorado Stratigraphy Chart. It reveals regional patterns of sediment accumulation and erosion through the past billion years. This chart’s linear timescale depicts depositional abutments, regionally extensive unconformities, lags, and karst surfaces; plots age-calibrated volcanoclastic units and illustrates stratigraphic gaps linked to major uplifts.

Colorado’s earliest sediments are clastic dikes in the Front Range crystalline rocks. These dikes are interpreted as Neoproterozoic relicts of strata that are completely eroded today. The oldest stratified sediments are in the Uinta Mountains, representing Cryogenian aulocogen sedimentation tied to the rifted western margin of Laurentia.

Lower Paleozoic strata are widespread and directly mantle much of Colorado’s basement. Deposited in shallow marine to near-shore conditions, they preserve a set of western thickening wedges interpreted to represent Sloss’ Sauk, Tippecanoe, and Kaskaskian sequences.

The Pennsylvanian-Permian Ancestral Rockies orogeny fostered deposition of thick marine, evaporite, and molasse deposits in narrow troughs and on the flanks of uptilted basement massifs. During the latest Paleozoic and early Mesozoic this orogenic terrain gave way to a landscape that, by the late Jurassic, was a low relief plain on which the Morrison Formation accumulated.

During the Cretaceous Sevier Orogeny, marine strata documenting the advent of the Western Interior Seaway are bracketed by transgressive mid Cretaceous Dakota Group and regressive Late Cretaceous Fox Hills Sandstone complexes, together with the non-marine cap of William’s Fork, Lance, and Laramie formations.

The Laramide Orogeny shattered Colorado’s landscape, producing a series of basement-cored uplifts that still define our topography. Thick synorogenic strata accumulated in basins between, and on mountain flanks adjacent to the uplifts. This orogenic landscape was again beveled by erosion, such that by Oligocene time high areas had been smoothed and basins filled to their brims with sediment.

During the Miocene, the region was uplifted and rivers draining Colorado eroded headward into the low relief terrain, sculpting the dramatic landscapes we enjoy today. This erosion continues to the present day.